Many viewers will ignore information that doesn't make sense to them. Web browsers and the Calibre epub editor will, for example, display a string of spaces as a single space and empty paragraphs or paragraphs with only space characters like these
<p> </p>
<p></p>
<p/>
are essentially ignored. They don't exist and, as you've discovered, won't add the blank line you want.
However, a paragraph containing a non-breaking space will work as desired. Replace any
<p class="ls" />
with
<p class="ls"> </p>
and you'll get a blank line. I've done it many times and KOReader handles it fine. On some systems the non-breaking space may be highlightable and selectable (like Calibre epub editor) or not (like my web browser on this forum).
Note: My post above of 18 minutes past the hour (or 48 minutes past the hour if you use a half hour time zone) was an addendum to an earlier post that I was told was going to go to a new user moderator a few hours ago. The first post has not appeared but the second one has, it's a mystery. I've tried to capture the essence of the missing post here in case it never reappears.